Let's talk a little bit about IOS commands for WAN trouble-shooting.
To verify the physcial cable connection on the routers, "show controller serial 1", where serial 1 is the serial port the cable attached to.
R1#show controller serial 1
For the sake of trouble-shooting, we may want to use command "show interface serial1" to gain more information about the interfaces. Sometimes we find that the physical interface is up and the line procol down, generally there could be two reasons
We forget to set the clock rate on DCE. The line protocal will be down after 30 seconds, because the DTE need to receive the clock rate to work correctly. To set the clock rate on the DCE, use command "clock rate 56000" to set the clock rate to, for instance, 56 kbps.
The encapsulation type mismatch on both ends of the connection.
For example, if we physically connected router R1 and R2, but set different encapsulation type on them.
Then, we will see physical interface up and line prococal down by running command "show interface serial1" on both router R1 and R2. To resolve the problem, we issue command "encapsulation hdlc" on R1 or issue command "encapsulation ppp" on R2, so that the encapsulation type matches on both ends of the serial link.